Conventionally, in the manufacturing process of a semiconductor device which requires a crystal film with a relatively large thickness as does a power device such as IGBT (Insulated Gate Bipolar Transistor), an epitaxial growth technique has been used which is to form a film by vapor-depositing a monocrystal thin film on a substrate such as a semiconductor wafer. A film forming apparatus used for the epitaxial growth technique is one example of a substrate processing apparatus, and in this film forming apparatus, a substrate such as a wafer is placed in a film forming chamber maintained at a normal pressure or a reduced pressure. Then, while this substrate is heated, gases as materials for film formation (material gases) are supplied into the film forming chamber. This causes thermal decomposition reaction and hydrogen reduction reaction of the material gases on a surface of the substrate, depositing an epitaxial film on the substrate.
In the film forming apparatus, a curvature measuring device (warp measuring device) is used for measuring the curvature of a substrate as an object to be measured. This curvature measuring device is mainly used for optimizing a process procedure or the like but is now used also in mass-production apparatuses, and constant warping monitoring is required increasingly. For example, during film formation of a gallium nitride (GaN) on an 8-inch silicon, monitoring of the degree of warping of a wafer during the film formation is quite important because of the presence of film forming conditions over a large temperature range in addition to a difference in thermal expansion coefficients and a large mismatch of crystal lattice constants between silicon and a GaN thin film. Neglecting this warp monitoring leads to decrease in product quality due to occurrence of breakage of the wafer or minute cracks in the thin film during film formation. Therefore, the warp monitoring is essential for optimization of a process procedure in advance of mass production but is also becoming necessary for keeping quality in mass-production situations where the state inside a film forming furnace changes little by little.
A currently common curvature measuring device irradiates two laser beams in parallel to a substrate via a window of the film forming chamber, detects the positions of two laser beams reflected by the substrate and returned via the window, reads the interval between them (incident position interval between the two laser beams), and converts it into the curvature of the substrate. As a method of detecting the laser beams at this moment, a 2-point CCD method is employed which is to detect two laser beams at once by a two-dimensional CCD (Charge-Coupled Device). Incidentally, the window passed by the laser beams is formed in a long shape extending in a diametrical direction of the substrate in the film forming chamber, in order to optically measure warp, temperature, and/or the like of the substrate. This window is called a view port, and for example, quartz is used as the material of the window.
However, the temperature in the film forming chamber becomes, for example, as high as about 1000° C. during film formation, and thus the window of the film forming chamber is exposed to high temperatures. At this moment, a large temperature difference occurs between the vicinity of edges of the window in contact with a wall of the film forming chamber and the center of the window, and thus a temperature gradient occurs in a short side direction of the long-shaped window. The refractive index of the window varies along the short side direction due to this temperature gradient in the short side direction. For example, while the refractive index of quartz increases as the temperature rises, temperature dependence of the refractive index varies depending on the material of the window, and thus the refractive index can become low as the temperature rises.
Therefore, when the incident points of the two laser beams on the window are aligned in parallel in the short side direction of the window, if a temperature gradient occurs in the short side direction of the window during film formation, the refractive indexes at these two incident points are different, and thus the incident position interval between the two laser beams varies due to the difference in the refractive indexes. That is, the incident position interval between the two laser beams varies due to a factor other than warping of the substrate, resulting in decreased accuracy of the curvature measurement.
An object to be solved by embodiments of the present invention is to provide a substrate processing apparatus capable of improving curvature measurement accuracy when the curvature of a substrate being processed is measured.